Field
The present application relates to the detection of the formation of clathrate hydrates and in particular, but not exclusively, to the measurement of pressures and corresponding temperatures at which clathrate hydrates are found to form.
Related Art
A clathrate is a chemical substance consisting of a lattice that traps or contains molecules. Clathrate hydrates (also referred to as gas clathrates, gas hydrates, clathrates, or hydrates) are crystalline water-based solids physically resembling ice, in which small non-polar molecules (typically gases) or polar molecules with large hydrophobic moieties are encapsulated or trapped inside “cages” of hydrogen bonded water molecules. In other words, clathrate hydrates are clathrate compounds in which the host molecule is water and the guest molecule is typically a gas or liquid. Most low molecular weight gases (such as oxygen, hydrogen, nitrogen, carbon dioxide, methane, hydrogen sulfide, argon, krypton, and xenon) as well as some higher hydrocarbons will form hydrates at suitable temperatures and pressures. Clathrate hydrates are not chemical compounds, as the sequestered molecules are never bonded to the lattice. The formation and decomposition of clathrate hydrates are first order phase transitions, not chemical reactions.
Clathrate hydrates are of interest in the petroleum industry, particularly with respect to producing, transporting and processing of natural gas and petroleum fluids. In these applications, the clathrate hydrates are typically composed of water and one or more of the following eight guest molecules: methane, ethane, propane, isobutane, normal butane, nitrogen, carbon dioxide, and hydrogen sulfide. Other guest molecules can include ethane, nitrous oxide, acetylene, vinyl chloride, methyl bromide, ethyl bromide, cyclopropane, methyl mercaptan, sulfur dioxide, argon, krypton, oxygen, xenon, trimethylene oxides, and others.
Clathrate hydrates can be a problem for development and production in the petroleum industry when working at high pressures and cold temperatures, possibly resulting in production delays and “blowouts” of wells. Gas hydrate formation during deepwater offshore drilling is a well-recognized operational hazard. At present, most common hydrate removal techniques include depressurization of the flow line, mechanical removal, and heating. Heat is supplied to raise the temperature in the section of the pipe where hydrate formation is expected. However, it is imperative to ensure uniform heating throughout the region of hydrate formation. In several cases, poorly controlled heating has caused release of gas (due to melting of hydrate), resulting in fatal accidents due to rapid increase in localized pressure in the pipe. Chemical additives have also been used to control the hydrate kinetics in order to prevent formation of hydrate nuclei and slow down their growth. These chemical inhibitors can cost millions of dollars for offshore wells.
The equilibrium clathrate hydrate formation point is the temperature (at a given pressure) or the pressure (at a given temperature) where the initial small quantity of clathrate hydrate appears after a sufficiently long time. This point corresponds to the thermodynamic formation point of clathrate hydrates. Laboratory measurements are conducted by forming some clathrate hydrate and then slowly heating or de-pressurizing the sample until it all dissociates. The point on the dissociation curve where no clathrate hydrate remains is identical to the thermodynamic formation point. In practice, there is a delay in forming clathrate hydrate until a lower temperature or higher pressure is reached. Before the thermodynamic formation point is reached hydrate cannot form. This is known as the stability limit. Beyond the stability limit clathrate hydrate can form but may not do so for a long time.
Most commercial clathrate hydrate phase equilibrium measurements are conducted in macroscopic systems which allow visual confirmation of clathrate hydrate formation and dissociation as the pressure or temperature is varied. A comprehensive review of the systems used for hydrate phase equilibrium measurements can be found in Sloan, E. D. and C. Koh, “Clathrate Hydrates of Natural Gases,” Third Edition, Taylor & Francis, 2007, pp. 320-326. These systems typically consist of a stainless steel container (hydrate cell) with a sight-glass, capable of withstanding high pressure. The cell is designed with different mechanisms to impart vigorous agitation on the fluids to enhance mixing between the gas and the liquid phase. The agitation is necessary for surface renewal and exposure of the liquid to the hydrate former. The agitation reduces the metastability during the clathrate hydrate formation stage. The source of agitation varies widely among the systems; namely, physical rocking of the hydrate cell, rotation, electromagnetic agitation, magnetic stirring.
Visual techniques can also be used to determine the temperature and pressure conditions of clathrate hydrate formation in reservoir fluids containing natural gas, carbon dioxide, hydrogen sulfide, gas condensate, or conventional oil in the presence of water. The visual techniques involve charging known amounts of hydrocarbon and aqueous fluids into the cells and then cycling the temperature over a range of values while maintaining a constant pressure. Visual detection of the formation and dissociation (melting) of clathrate hydrate crystals during heating and cooling is used to determine the clathrate hydrate formation conditions.
Clathrate hydrate equilibrium measurements can also be conducted in blind cells (autoclaves) based only on pressure-temperature trace. Mechanical mixing is provided by a magnetically coupled impeller inside the cell and controlled by an external motor. Several mesoscopic methods are also available for clathrate hydrate phase measurement such as methods that employ a light scattering/reflectance method and synthetic porous media. In addition to measuring clathrate hydrate equilibrium, the light scattering method also measures the changes in the hydrate particle size during clathrate hydrate formation and dissociation. The synthetic porous media method can use a glass micro-model to demonstrate that clathrate hydrate formation in porous media can occur at the gas-liquid interface and also from dissolved gas in the liquid.
The clathrate hydrate equilibrium measurement methods produce accurate measurements supported by thermodynamic models. However, the equipment and setup for these methods are designed for use in laboratory environments. Due to the large volume of the test samples (several hundred cc), the metastable period prior to detectable clathrate hydrate formation is usually long (hours to days). Additionally, the handling of large sample volumes at high pressure poses potential hazards.